Get all site collections in the WebApplication using a Object Model SharePoint

  public void GetAllSiteCollections(string url)
        {
            SPSecurity.RunWithElevatedPrivileges(delegate()
            {
                SPWebApplication webApplication = SPWebApplication.Lookup(new Uri(url));
                foreach (SPSite site in webApplication.Sites)
                {
                  //do your action
                }
            });
        }